Pseudomonas aeruginosa (P. aeruginosa) is an opportunistic pathogen that causes acute infections, primarily in association with tissue injuries. P. aeruginosa forms biofilms on indwelling devices and on the pulmonary tissues of patients with the genetic disorder, cystic fibrosis. Biofilm infections are difficult to treat with conventional antibiotic therapies. However, research has demonstrated that iron is essential for proper biofilm formation by P. aeruginosa, and therefore iron-uptake systems are potential targets for anti-Pseudomonas therapies.
P. aeruginosa is able to scavenge iron from the host environment by using the secreted iron-binding siderophores, pyochelin and pyoverdine. Pyoverdine (Pvd) is a peptide-linked hydroxamate- and catecholate-type ligand, and pyochelin (Pch) is a derivatized conjugate of salicylate and two molecules of cysteine and having phenol, carboxylate, and amine ligand functionalities. Both Pvd and Pch have demonstrated roles in P. aeruginosa virulence with some indication of synergism. Double-deficient mutants unable to make either siderophore are much more attenuated in virulence than either single-deficient mutant unable to make just one of the two siderophores (Takase et al., Infection and Immunity, April 2000, p. 1834-1839). Furthermore, pyoverdine acts as a signalling molecule to control production of several virulence factors as well as pyoverdine itself; while it has been proposed that pyochelin may be part of a system for obtaining divalent metals such as ferrous iron and zinc for P. aeruginosa's pathogenicity, in addition to ferric iron (Visca et al., 1992).
Three structurally different pyoverdine types or groups have been identified from several P. aeruginosa strains: from P. aeruginosa ATCC 15692 (Briskot et al., 1989, Liebigs Ann Chem, p. 375-384), from P. aeruginosa ATCC 27853 (Tappe et al., 1993, J. Prakt-Chem., 335, p. 83-87) and from a natural isolate, P. aeruginosa R (Gipp et al., 1991, Z. Naturforsch, 46c, p. 534-541). Moreover, comparative biological investigations on 88 clinical isolates and the two collection strains mentioned above revealed three different strain-specific pyoverdine-mediated iron uptake systems (Cornells et al., 1989, Infect Immun., 57, p. 3491-3497; Meyer et al., 1997, Microbiology, 143, p. 35-43) according to the reference strains: P. aeruginosa ATCC 15692 (Type I Pvd or Pvd I), P. aeruginosa ATCC 27853 (Type II Pvd or Pvd II) and the clinical isolates P. aeruginosa R and pa6 (Type III Pvd or Pvd III).
Each pyoverdine type has three members (subtypes) differing in the side chain which is succinyl (s), succinamide (sa) or α-ketoglutaryl (αKG), namely, Pvd type I succinyl, Pvd type I succinamide, Pvd type I α-ketoglutaryl, Pvd type II succinyl, Pvd type II succinamide, Pvd type II α-ketoglutaryl, Pvd type III succinyl, Pvd type III succinamide and Pvd type III α-ketoglutaryl.
Each P. aeruginosa strain expresses one Pvd type i.e. P. aeruginosa ATCC 15692 expresses Type I Pvd, P. aeruginosa ATCC 27853 expresses Type II Pvd and P. aeruginosa R and pa6 expresses Type III Pvd, whereby each Pvd type includes all three members of the respective type, and each said strain also expresses pyochelin.
The inventors have identified the pyoverdines and pyochelin as targets which are crucial for P. aeruginosa's pathogenicity and developed specific inhibitors for such targets, i.e. for each type of Pvd including for every type the three members (subtypes) differing in the side chain (Pvd I s, Pvd I sa, Pvd I αKG, Pvd II s, Pvd II sa, Pvd II αKG, Pvd III s, Pvd III sa, Pvd III αKG) as well as for Pch, and in every case to the free siderophore as well as to the siderophore with bound iron without creating the strong selective pressure imposed by conventional antibiotics (see also EP 15 305 242.8).
The biofilm mode of growth is believed to be critical for persistent P. aeruginosa infections (Costerton et al., 1999; Singh et al., 2000) and the dual expression of Pvd and Pch genes is necessary for normal biofilm development (Banin et al., 2005). Given that P. aeruginosa produces an impressive array of virulence factors, all playing a role in its pathogenicity, a strategy to efficiently inhibit P. aeruginosa virulence is to target several virulence factors.